Water Monitoring and Sampling

Monitoring water on site

The monitoring of water quality and flows on site is extensive, and it includes:

  • mine water from No. 7 shaft in the open pit
  • runoff from pit walls
  • water from underground at Favona and Trio operations
  • decant water
  • collection pond water
  • silt pond water
  • individual drains around the waste rock embankments
  • water treatment plant monitoring
  • river water and aquatic biology.

This data is integral to the daily management of the site. It can affect, for example, decisions made about water types to be treated through the water treatment plant. While much of the monitoring is required by the consent conditions, regular monitoring is recognised as being good practice. It allows the formation of a database so that trends can be monitored, and any unusual results can be investigated.

Long term monitoring data also provides confirmation that the mixing and modelling studies carried out for the lake and the tailings ponds continue to be accurate. Monitoring of water quality and flows from the individual drains beneath the tailings storage facilities provides a good indication of their performance over time, allowing predictions of likely future performance to be made. This is an important issue for mine closure.

Monitoring of rivers and streams

Monitoring of waterways, particularly the Ohinemuri River, Ruahorehore, Mangatoetoe and Eastern Streams is an important part of the operation. This monitoring is not only to comply with the consent conditions, but also to provide an indication of the baseline conditions that existed prior to mining. In the case of the Mangatoetoe Stream, baseline monitoring has been carried out to assess the effects of the overflow of the pit lake. The collection of data over a long period of time provides a good indication of seasonal changes and trends that may be developing in the river system.

Baseline studies of the water quality and aquatic biology of the Ohinemuri River and its tributaries commenced in 1981, and these indicated that the quality of the river and stream habitats was largely determined by the land use within the respective catchments. This situation is commonly found in rivers and streams throughout New Zealand where the waters flow from bush covered catchments downstream through open farmland.

In the headwaters of streams located in native bush, the water quality was found to be good and the streams supported a diverse range of aquatic plants, benthic macroinvertebrates and fish. In the lower reaches of the tributaries and in the Ohinemuri River the tendency was for:

  • water quality to decrease slightly downstream and under higher flows
  • aquatic plant (macrophytes and algae) cover to increase
  • diversity of the benthic macroinvertebrates to decrease and abundance to increase
  • fish habitat quality to decrease.

These changes were considered to be due primarily to the removal of native bush and conversion to farmland. This resulted in increased nutrient and sediment input to the waterways. Riparian willows, which provided cover for fish, shaded the river and maintained lower water temperatures, were also removed by the council. Further changes were caused by the discharge of waste from cowsheds, which increased the organic and nutrient loading to the river and its tributaries.
See how Wild about Waihi (WaW) helps to redress these issues.

In summary the development of approximately 65% of the catchment for farmland, and willow clearance (prior to 1989) along the tributaries and main stem of the Ohinemuri River resulted in a general lowering of the water quality of the Ohinemuri River and changes to the habitat quality of the river. Catchment development resulted in increased inputs of dissolved nutrients and suspended solids (sediment). The clearance of willows reduced instream cover for aquatic organisms, raised water temperatures and increased the potential for algae and other aquatic plants to grow in the river.

Operational monitoring

Monitoring for water quality, fish, periphyton, sediments, and benthic macroinvertebrates is carried out at a number of sites both upstream and downstream of the discharge points. The characteristics of the river are compared to those that existed prior to the discharge of treated water.

Monitoring to date indicates that the treated water discharge has had no significant adverse effect on the Ohinemuri River.

Collecting water for sampling

Chemical analysis of water is an exacting procedure. The use of extremely sophisticated instruments and procedures means that some elements can be detected to amazingly low concentrations. Only a few years ago this level of detection was not available, and consequently the standards set for water quality determination are always changing.

Data collection and analysis involves a chain of responsibilities, and the data will only be as good as the weakest link in the chain. The accurate collection of data is the first and most important link in this chain. It is essential that contamination of the sample does not occur during sample collection, or at any other time. To ensure that accuracy and precision goals are met the following precautions are taken:

  • water is collected upstream of the person sampling, from a specified position in the flow
  • latex gloves are worn to prevent chemicals that are naturally present on the skin, or from deposits left on hands from other activities, from washing into the sample bottle; for example, cigarette smoking leaves a cyanide deposit on smoker’s hands and this level of contamination can easily be detected by modern analytical equipment
  • chemical stability of sample water is maintained by specific preservatives, e.g., nitric acid is used in samples collected for metal ion analysis
  • the sampling procedure includes the use of controls, blanks and replicate samples for quality control
  • samples are quickly chilled to slow down reactions that could change the nature of the sample in the time between sample collection and analysis
  • samples are sent by courier, immediately after collection, to independent laboratories for analysis
  • all procedures are carefully documented, including the ‘chain of custody’, in accordance with the quality assurance programme; Australian/New Zealand Standard procedures are followed at every step of the sample collection and handling process (as outlined in AS/NZS 5667.1-6: 1998).